The present invention relates generally to operations wherein wellheads are installed on subterranean wells and, in a preferred embodiment thereof, more particularly provides a wellhead plug and associated methods of plugging wellheads.
With the advent of horizontal tree designs for use in offshore well completions, the safety, convenience, and economics of wellhead installations have been greatly improved. These enhancements are detailed in a paper (OTC 7917) presented at the 27th Annual Offshore Technology Conference in 1995. That paper is incorporated herein by this reference.
A major benefit derived from the use of a horizontal tree is that a wireline-retrievable plug may be used in a vertical portion of the wellhead above the horizontally-oriented production flow passage. The plug may be removed from the wellhead after the well has been placed in production, for example, in order to provide access to the well for workover operations. Typically, the plug is set in a profile formed internally on a tubing hanger installed in the wellhead before the well was placed in production.
In some circumstances, a second plug may be installed above the plug set in the tree. The upper plug is typically set in a high pressure cap installed on top of the wellhead.
Since the cost of the wellhead is related to its vertical length, it may be easily seen that it is desirable for the wellhead plugs to be made as short as possible, while maintaining their ability to withstand fluid pressure applied from above or below. Costs are also associated with installation and retrieval of the plugs, and the most economical installations and retrievals are typically performed by wireline or slickline.
In many instances, a well may be in production for many years before the need arises to perform workover operations, or to otherwise gain physical access to the wellbore. Thus, a wellhead plug may be called on to maintain its sealing capability for many years as well. For this reason and others, it is desirable for the plug to be fitted with metal-to-metal (MTM) seals, which are exceptionally long-lasting and resistant to long term temperature degradation. Unfortunately, the use of MTM seals typically requires highly polished seal surfaces, application of special coatings and/or metal alloys to seal surfaces, very close machining tolerances, and/or other special operations, which make it rather expensive to utilize MTM seals.
In any event, when MTM seals are utilized it is important to prevent relative movement between surfaces contacted by the MTM seals. This is because both the seals and surfaces are made of metal and will become damaged if there is relative movement between them while they are in contact and under pressure. If such damage occurs, the MTM seals will no longer function properly. Therefore, in operation, when MTM seals are utilized on wellhead plugs, it is desirable for the plugs to be "static", that is, motionless relative to the wellhead in which the plugs are installed.
Additionally, it is desirable for wellhead plugs to be maintained in their set positions for long periods of time, since, as set forth above, many years may elapse before they are removed to provide access to the wellbore. Various schemes have been developed for maintaining a wellhead plug in its set position, while still permitting convenient unsetting by wireline operations. For example, some plugs rely on gravity and friction between an expander sleeve and keys to maintain the plugs in their set position. These, however, have been less than satisfactory because vibration and stress cycles tend to dislocate the expander sleeve over time.
It is also desirable for a wellhead plug to provide multiple locations to aid in fishing operations when the plug cannot be retrieved by normal wireline operations as designed. Again, this is of particular importance in wellhead plugs, since they may be retrieved after many years of service, and since they control physical access to the well.
And finally, typical wellhead plugs utilize a downward jarring force to displace the expander sleeve and thereby force the keys outward into the profile in the tubing hanger or high pressure cap. This jarring force is difficult to control accurately and may result in improperly set plugs, damaged plugs, and/or misruns, etc. It would be much more desirable to provide a wellhead plug which is settable by application of fluid pressure. Such methods of setting wellhead plugs would be of particular value where MTM seals are utilized with the plugs.
From the foregoing, it can be seen that it would be quite desirable to provide a wellhead plug capable of utilizing MTM seals, which is static in operation, which is wireline-conveyable and wireline-retrievable, which is settable by application of fluid pressure, which includes features which reliably maintain the plug in its set position, which is relatively short in length and small in diameter, and which provides multiple locations for fishing operations. It is accordingly an object of the present invention to provide such a wellhead plug and associated methods of plugging wellheads.